optics, refraction and refractive errors

Optics, Refraction and

Refractive Errors

DR M.FEIZI

IN THE NAME OF ALLAH

A ray of light is an extremely narrow

beam of light.

All visible objects emit or reflect light rays in all directions.

Our eyes detect light rays.

converge: come together

Images are formed when

light rays converge.

Reflection(bouncing light)

Reflection is when light changes direction by

bouncing off a surface.

When light is reflected

off a mirror, it hits the mirror at the same

angle (θi, the incidence

angle) as it reflects off

the mirror (θr, the

reflection angle).

The normal is an

imaginary line which lies

at right angles to the

mirror where the ray hits it.

θr θi

Mirror

normal

incident ray

reflected ray

Refraction(bending light)

Refraction is when light bends as it

passes from one medium into

another.

When light traveling through air

passes into the glass block it is

refracted towards the normal.

Refractive index =

Speed of light in vacuum

Speed of light in media

normal

normal

air

air

θr

θi

θr

θi

glass

block

Convex Lenses

Convex lenses are thicker in the middle and focus light rays to a focal point in

front of the lens.

The focal length of the lens is the distance between the center of the lens and

the point where the light rays are focused.

Concave Lenses

Concave lenses are thin in the middle and

make light rays diverge (spread out).

If the rays of light are traced back (dotted sight

lines), they all intersect at the focal point (F)

behind the lens.

optical axis

•F

Refractive Errors

Emmetropia

Ametropia

Emmetropia

Emmetropia means no

Refractive error

It is the ideal condition in

which the incident parallel

rays come to a perfect

focus upon the light

sensitive layer of the retina,

When accommodation is

at rest

Ametropia

Ametropia means Refractive error Eye

It is the opposite condition , wherein the parallel rays of light are not

focused exactly upon the retina , When the accommodation is at rest

Ametropia

Myopia

Hypermetropia

Astigmatism

Myopia

Principal focus is formed in front of the retina

Causes

Axial Myopia

Curvature Myopia

Index Myopia

Abnormal position of the lens

Axial Myopia

Axial myopia results from increase in anteroposterior length of the

eye ball.

Normal Axial length- 23mm to 24mm

1mm increase in AL – 3Ds of Myopia

Curvature Myopia

Curvatural myopia occurs due to increased curvature of the cornea

and Lens or both.

Anterior surface of the cornea- 7.8mm

Posterior surface of the cornea- 6.5mm

1mm decreases in radius of curvature results in – 6 Ds of Myopia

Index myopia

Index myopia results from increase in the refractive index of

crystalline lens.

Refractive index of normal Lens - 1.42

Types

Congenital myopia

Simple Myopia (or) Developmental myopia

Pathological Myopia (or) Degenerative myopia

Acquired myopia

Congenital myopia

Congenital myopia is present since birth however, it is usually

diagnosed by the age of 2 – 3 years.

Simple myopia

Simple or developmental myopia is the commonest variety. It is

considered as a physiological error not associated with any disease

of the eye.

Power limit less than 6D

Aetiology

Axial type of simple myopia

Curvatural type of simple myopia

Pathological myopia

Myopia associated with degenerative changes in the eye.

Myopia more than 6D to25D or More than 25D

Aetiology

Axial growth

(i) Heredity

(ii) General growth process

Symptoms

Poor vision for distance( even near)

Asthenopic symptoms

Exophoria

Signs

Large eye ball

deep Anterior chamber

sluggish Pupil

Large Disc

Complications

Retinal tear – Vitreous haemorrhage

Retinal detachment

Degeneration of the vitreous

Primary open angle Glaucoma

Posterior cortical cataract

Posterior staphyloma

Treatment

Optical

Spectacle Correction (Concave Lens)

Contact lens

Surgical

PRK

Epikeratophakia

Redial Keratotomy

Optical Treatment

Concave lens

Myopic with Exophoria give full correction.

Myopic with Esophoria give under

correction.

Hypermetropia

Principal focus is formed behind

the retina

Causes

Axial Hypermetropia

Curvature Hypermetropia

Index Hypermetropia

Abnormal position of the lens

Axial Hypermetropia

Axial hypermetropia is by far the commonest

In fact, all the new- borns are almost invariably

hypermetropic (approx,+2.50D) This is due to

shortness of the globe, and is physiological.

Normal axial length – 23mm to 24mm

1mm decrease in AL – 3Ds of hypermetropia

Curvature Hypermetropia

In which the curvature of cornea, Lens or both is flatter than the normal resulting in a decrease in the refractive power of the eye.

Anterior surface of the cornea- 7.8mm

Posterior surface of the cornea- 6.5mm

1mm increase in radius of curvature results in –6Ds of hypermetropia

Index Hypermetropia

Index hypermetropia occurs due to change in refractive index of

the lens in old age. It may also occur in diabetics under treatment.

Refractive index of Normal Lens - 1.42

Classification

Total Hypermetropia may be divided into

(a) Latent Hypermetropia

(b) Manifest Hypermetropia

(i) Facultive Hypermetropia

(ii)Absolute Hypermetropia

Latent Hypermetropia

LH which is corrected physiologically by the tone of ciliary muscle.

As a rule latent hypermetropia amounts to only one dioptre. It can

be revealed only after atropine cycloplegia.

Manifest Hypermetropia

MH is made up of two components

Facultative hypermetropia is that

part of hypermetropia which can be corrected by the effort of accommodation.

Absolute hypermetropia which can not

be overcome by the effort of accommodation.

Clinical Types

Simple hypermetropia

Pathological hypermetropia

Functional hypermetropia

Simple hypermetropia

It results from normal biological variation in the development of the

eye ball.

It includes Axial and Curvatural HM

It may be hereditary.

Pathological hypermetropia

PH results due to either congenital or acquired conditions of the eye

ball which are out side the normal biological variations of the

development.

The Normal Age Variation

At birth:- 2D to 3 D Commonly Present

At the age of 5 Yrs- 90% of Children’s are Hypermetropic

At Puberty:- Emmetropic

Symptoms

Head ache

Blurred vision particular near work

Convergent squint

Early onset of presbyopia

Eye Strain

Complications

Eye appears to be small including

cornea and anterior chamber

becomes shallow

Extreme cases – Microphthalmos

Treatment

Optical

Spectacle ( Convex Lens )

Contact lens

Hypermetropic with Exophoria give under correction

Hypermetropic with Esophoria give full correction

Surgical

Thermokeratoplasty

Astigmatism

Astigmatism is that condition of

Refraction where there are two point focus

of light

Causes

Curvature

Ex: Keratoconus, Lenticonus etc..

Centering error

Ex: Sub location of the lens

Refractive index

Ex: Cataract

Retinal

Oblique placement of macula

Types

Regular

Irregular

Regular astigmatism

Refractive types

Physiological types

Refractive types

Simple astigmatism

Compound astigmatism

Mixed astigmatism

Physiological types

With rule astigmatism

Against rule astigmatism

Oblique astigmatism

Symptoms

Head ache

Blurring of vision

Eye tired

Eye ache

Head Tilt

Half-closure of the lids (High astigmatism)

Blurring & Itching (Low astigmatism)

Treatment

Optical Treatment

* Cylindrical lens

* Under correction

* Contact lens (RGP, Toric)

Refractive surgery

* Astigmatic Keratotomy

* PRK, LASIK

Study Reports

Percentage of astigmatism

* 0.25-0.50D 50%

* 0.75-1.00D 25%

* 1.00-4.00D 24%

*>4.00D 1%

Percentage of Types

* with rule 38%

* Against rule 30%

* Oblique 32%

Presbyopia

This is a physiological aging

process, In which the near

point gradually recedes

beyond the normal reading

or working distance

Causes

Lens matrix is harder and less easily moulded

Lens capsule is less elastic

Progressive increase in size of the lens

Weakening of the ciliary muscle

Symptoms

Patient holds the book at arms length

Patient prefers to read in bright light

Eye strain

Head ache

Eyes feels tired and ache

Treatment

Convex lens

Methods of prescription

* Occupation

* Working distance

* Age

Surgical

* Anterior ciliary sclerotomy

* Laser thermal keratoplasty

* Small diameter corneal inlays

Aphakia

Aphakia means absence of

the Crystalline lens from the

Eye ball

Causes

Congenital

Surgery

Traumatic

Optics of Aphakia

Anterior focal distance – 23mm (N-15mm)

Posterior focal distance- 31mm (N-24mm)

The Nodel point of the eye is thus moved forward

Strong converging (convex) lens- +10D

Signs

Anterior chamber – Deep

Iris

(i) Iridodonesis (or) Tremulousness

(ii) Peripheral button-hole iridectomy mark

Pupil - Jet black reflex

Retinoscopy – reveals high hypermetropia and astigmatism

Disadvantages

Image magnification of about 25-30%

Roving ring scotoma (The scotoma extents

from 50°- 65° from central fixation)

Restriction of the visual field

Coloured vision

Inaccurate spectacle correction because of errorneous vertex distance

Treatment

Spectacle ( Convex lens )

Contact lens

Secondary IOL

Epikeratophakia

Keratophakia

Pseudophakia

Pseudophakia means False

lens

Image magnification

Advantages

Image magnification is only 0- 2%

Minimum (or) No Anisokonia with rapid return of

binocularity

Normal Peripheral field

Freedom from handling of the optical devices

Cosmetically it is well accepted

Disadvantages

Risks and complications may be more

Initially, the cost is more

PCO

CME

IOL related complications

Thank “U”